ABSTRACT Defective proteins result from mutations, mistakes of transcription, stress, and other factors. These aberrant proteins are often toxic and cause a number of human diseases. There are several quality control (QC) pathways in the cells: nonsense-mediated, no-stop, and no-go decays for defective mRNAs, and the ubiquitin/proteasome system for already synthesized misfolded proteins. Recently, we discovered a new QC pathway called Regulation of Aberrant Protein Production (RAPP). RAPP is a preemptive QC, it monitors proteins during their synthesis at the ribosome, senses defective proteins and degrades their mRNA templates. It is the first example of transferring information about aberrant proteins to mRNA degradation machinery. Normally, nascent polypeptides emerged from the ribosome exit tunnel interact with targeting or folding factors. When a mutation prevents these interactions, the Ago2 protein, a sensor in the RAPP response, detects the loss of these important interactions and triggers mRNA degradation. Surprisingly, Ago2 endonuclease activity is not required in the process. Thus, the molecular mechanism of RAPP is not well understood, very little is known about its mRNA degradation machinery and its substrates. The proposed project will fill these gaps in knowledge. Our hypothesis is that RAPP is a general protein quality control pathway that consists of three major steps: detection of an aberrant nascent chain complex, translational repression, and formation of specialized cytoplasmic foci for degradation of the aberrant mRNA. Our specific aims are designed to test this hypothesis and directed to (1) elucidate the mechanism by which protein expression is down-regulated in RAPP, and (2) determine whether RAPP is a general mechanism of protein quality control for secretory and membrane proteins in mammalian cells. The proposal involves application of comprehensive in vivo and in vitro approaches, including our unique technology, iPINCH, for identification of Proteins Interacting with Nascent Chains. Determining the scope and mechanism of RAPP will impact the field of protein quality control research by providing a better understanding of cellular defense against erroneous and potentially toxic proteins.